The present invention relates to thick film and thin film electrical resistors, and particularly to resistors of those types which can be trimmed to a required resistance with improved accuracy.
It is well known that the resistance of film resistors can be increased, from a low initial value provided intentionally, when a film of resistive material is deposited on a substrate, to a higher final value which is desired for the use of such a film resistor in a circuit, by removing a portion of the resistive material through the use of a laser trimmer. Because of the inaccuracies inherent in manufacturing printed film resistors, particularly thick film resistors often used in hybrid microelectronic circuits, individual resistors in an array printed as a unit may initially be as much as 50% low in resistance, in order to ensure that none of the resistors manufactured in the array has too high a resistance to be useable. After such resistors are printed onto a substrate as a group they are trimmed individually to increase the resistance of each resistor to the required value. Trimming is accomplished, typically, by the use of a controlled stream of mechanically abrasive particles, or a laser beam, under automatic computer control to cut away a narrow strip of resistive material. This has the effect of narrowing or elongating the path for current through the resistor, or both narrowing and elongating the path. It is desirable for the sake of economy to provide a film resistor which can be trimmed to the required value in a minimum amount of time and with a minimum amount of trimming. Of course, the value of the trimmed resistor must be accurate within the required tolerances.
There are several problems involved with trimming of resistors as conventionally accomplished using automatic trimmers. In using a simple plunge cut to trim previously available film resistors, the rate of change of resistance for a given additional extension of a trimming cut increases rapidly, particularly as the plunge cut approaches an opposite margin of a film resistor. Thus, for a physically small resistor which initially had a significantly low resistance requiring a long trim cut, the value of the trimmed resistor may differ from the desired final resistance by a wider margin than is desirable. That is, since an abrasive stream or a laser beam is finite in size the smallest amount of change of resistance which can be accomplished under computer control using a simple plunge cut in a conventional film resistor may exceed the allowable range of values of the resistor when the plunge cut approaches the opposite margin of the resistive film of the resistor, when the initial resistance was quite low. It has been a rule of thumb, therefore, that a plunge cut should not be used to increase a resistor's initial value more than 100%.
Another problem with previously available film resistors is that significant amounts of trimming may reduce the power capacity of a film resistor below the minimum required, which is to say that the current density may be too high at some points in the portions of the film of resistive material remaining after trimming. Additionally, a long plunge cut may even reach an opposite edge of a film of resistive material, severing the resistor and rendering worthless the entire printed circuit of which the resistor is a part. Limiting the length of a plunge cut in order to limit current density may require a second trim cut (a "shadow cut") alongside the plunge cut to provide the remaining amount of trimming of the resistor without reducing current capacity unacceptably. Making such a shadow cut increases the amount of time required to trim a resistor, resulting in higher manufacturing costs for the circuit.
Preferably, a resistor should be able to be trimmed by a single rectilinear, or plunge, cut which can be accomplished quickly and accurately using a computer controlled trimmer. In the past, however, automatic control of a trimmer has required that the speed of advance of an abrasive or laser trimming beam be reduced, in order to provide the required accuracy in the final resistance value. This reduction of trimming speed also increases the required time and thus the expense of resistor trimming.
Other patterns for cutting the resistive film of such a resistor to trim the resistance are more complex to control by computers and take more time than a plunge cut to provide the required trimming of the resistor.
What is needed, then, is a film resistor which can be manufactured economically using known technology, and a method for trimming such a resistor accurately and quickly over a wider range of resistance values than has previously been possible, to provide a final value of resistance which is dependably accurate within required tolerances upon completion of trimming. Preferably, such a resistor and method should also eliminate resistor destruction resulting from the trimming process.